Semiconductor device and method for fabricating the same

ABSTRACT

A metal target, at least the surface region of which has been oxidized, is prepared in a chamber. Then, a sputtering process is performed on the metal target with an inert gas ambient created in the chamber, thereby depositing a first metal oxide film as a lower part of a gate insulating film over a semiconductor substrate. Next, a reactive sputtering process is performed on the metal target with a mixed gas ambient, containing the inert gas and an oxygen gas, created in the chamber, thereby depositing a second metal oxide film as a middle or upper part of the gate insulating film over the first metal oxide film.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor device includinga gate insulating film made of a material with a high dielectricconstant (which will be herein referred to as a“high-dielectric-constant material”) and also relates to a method forfabricating the device.

[0002] Recently, there has been a growing demand forhigh-speed-operating semiconductor devices. To meet this demand, thegate insulating film of MOSFETs has been further thinned for the purposeof increasing the drivability thereof.

[0003] However, if the gate insulating film is a thin film of SiO₂ (witha relative dielectric constant ε of 3.9), the gate leakage currentincreases noticeably because a tunneling current flows therethrough.

[0004] Thus, to prevent the gate leakage current from increasing whileenhancing the drivability of MOSFETs, various methods for increasing theactual thickness of the gate insulating film and obtaining a desiredgate capacitance have been researched. For example, according to one ofthose methods, the gate insulating film is made of ahigh-dielectric-constant material (high-κ material) such as HfO₂(hafnium dioxide with a relative dielectric constant ε of about 30) orZrO₂ (zirconium dioxide with a relative dielectric constant ε of about25).

[0005] To deposit a gate insulating film of a high-dielectric-constantmaterial, a reactive sputtering process is performed using a target ofHf or Zr, for example, in a mixed gas ambient containing Ar (argon) andO₂ gases, for example. In this manner, a gate insulating film of ahigh-dielectric-constant material such as HfO₂ or ZrO₂ can be depositedover a semiconductor substrate.

[0006] However, if the gate insulating film of thehigh-dielectric-constant material is deposited over a silicon substrateby the reactive sputtering method, for example, the surface of thesilicon substrate is oxidized by a plasma created from the O₂ gas duringthe reactive sputtering process. Thus, an unwanted silicon dioxide filmis formed between the silicon substrate and gate insulating film. Itshould be noted that the unwanted film will be herein referred to as a“silicon dioxide film” but can actually be any other silicon oxide filmwith a non-stoichimetric composition. Consequently, the gate insulatingfilm becomes a stack of the silicon dioxide film with a relatively lowdielectric constant and the high-dielectric-constant film. As a result,the gate insulating film has its effective dielectric constant decreasedas a whole.

[0007] That is to say, the known method for fabricating a semiconductordevice cannot obtain the desired gate capacitance. Thus, it is difficultto enhance the drivability of MOSFETs.

[0008]FIG. 7 is a cross-sectional view showing the known method forfabricating a semiconductor device.

[0009] As shown in FIG. 7, a target 80 of Zr is placed in a chamber (notshown) and a silicon substrate 90 is loaded thereto. Then, a reactivesputtering process is performed using the target 80 with a mixed gasambient containing Ar and O₂ gases created in the chamber. During thisprocess, the surface of the target 80 is oxidized, thereby forming a Zroxide layer 81 thereon. At the same time, the surface of the siliconsubstrate 90 is also oxidized to be covered with a silicon dioxide film91. Further, as a result of the reactive sputtering process, a Zr oxidefilm 92 is formed over the silicon substrate 90 with the silicon dioxidefilm 91 interposed therebetween. Accordingly, the resultant gateinsulating film becomes a stack of the silicon dioxide film 91 and Zroxide film 92. As a result, the gate capacitance decreases compared to agate insulating film that has the same thickness but consistsessentially of a Zr oxide film alone.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to enhance thedrivability of MOSFETs by getting a gate insulating film, consistingessentially of a high-dielectric-constant material alone, formed by asputtering process without allowing any silicon dioxide film to exist onthe surface of a semiconductor substrate.

[0011] An inventive method for fabricating a semiconductor deviceincludes the steps of: a) preparing a metal target in a chamber, atleast a surface region of the target having been oxidized; b) performinga sputtering process using the metal target with an inert gas ambientcreated in the chamber, thereby depositing a first metal oxide film as alower part of a gate insulating film over a semiconductor substrate; andc) performing a reactive sputtering process on the metal target with amixed gas ambient, containing the inert gas and an oxygen gas, createdin the chamber, thereby depositing a second metal oxide film as a middleor upper part of the gate insulating film over the first metal oxidefilm.

[0012] According to the inventive method, in the step of depositing afirst metal oxide film over a semiconductor substrate, i.e., the initialstage of a process for forming a gate insulating film, no reactivesputtering process is performed but a sputtering process is performedusing a metal target, at least the surface region of which has beenoxidized, in an ambient containing no oxygen gas. Thus, the first metaloxide film can be deposited over the semiconductor substrate withoutallowing any silicon dioxide film to exist on the surface of thesemiconductor substrate. Also, in the step of depositing a second metaloxide film over the first metal oxide film, i.e., after the initialstage of the process for forming the gate insulating film is over, areactive sputtering process is performed in an ambient containing anoxygen gas with the surface of the semiconductor substrate covered withthe first metal oxide film. Thus, the second metal oxide film can bedeposited over the first metal oxide film without allowing any silicondioxide film to exist on the surface of the semiconductor substrate.Accordingly, the gate insulating film can be essentially made up of thefirst and second metal oxide films alone. In other words, a gateinsulating film consisting essentially of a high-dielectric-constantmaterial alone can be formed. As a result, the resultant MOSFET can haveits gate capacitance increased and its drivability enhanced. Inaddition, a gate leakage current can be minimized because the gateinsulating film can be thick enough with a desired gate capacitancemaintained.

[0013] In one embodiment of the present invention, the step a) mayinclude the step of performing a provisional reactive sputtering processon the metal target to be oxidized with a mixed gas ambient, containingthe inert and oxygen gases, created in the chamber, thereby oxidizingthe surface region of the metal target before the semiconductorsubstrate is loaded into the chamber.

[0014] Then, the metal target with the oxidized surface region can beprepared easily.

[0015] In this particular embodiment, the provisional reactivesputtering process is preferably performed on another semiconductorsubstrate that has been loaded into the chamber before the step a) isstarted.

[0016] Then, no insulating metal oxide is deposited on a wafer stage(which will be used as a gas-discharge electrode during the subsequentsputtering process steps) in the chamber when the surface region of themetal target is oxidized. As a result, it is possible to avoid theinability to apply a voltage to the semiconductor substrate in thesubsequent process steps.

[0017] In another embodiment, the step c) may include the step ofintroducing the oxygen gas into the chamber with the inert gas, used inthe step b), left in the chamber and with a gas-discharge continued fromthe step b) to carry out the reactive sputtering process.

[0018] Then, the steps b) and c) of depositing the first and secondmetal oxide films can be performed continuously. As a result, thethroughput of the process improves.

[0019] In an alternative embodiment, the inventive method may furtherinclude, between the steps b) and c), the step of introducing the oxygengas into the chamber with the inert gas, used in the step b), left inthe chamber and with a gas-discharge for the sputtering processsuspended.

[0020] Then, the mixture ratio of the inert and oxygen gases can befixed before the step c) of depositing the second metal oxide film isstarted. As a result, the oxygen concentration of the second metal oxidefilm is controllable more easily.

[0021] In another alternative embodiment, the inventive method mayfurther include, between the steps b) and c), the step of exhausting theinert gas, used in step b), from the chamber and then newly introducingthe inert gas along with the oxygen gas into the chamber with agas-discharge for the sputtering process suspended.

[0022] Then, the mixture ratio of the inert and oxygen gases should befixed before the step c) of depositing the second metal oxide film isstarted. As a result, the oxygen concentration of the second metal oxidefilm is controllable much more easily.

[0023] In yet another embodiment, the step c) may include the step ofsupplying the oxygen gas at a controlled flow rate into the chamber todeposit the second metal oxide film with a different oxygenconcentration from that of the first metal oxide film.

[0024] Then, the structure of the gate insulating film can be optimizedwith the reliability and the dielectric constant of the gate insulatingfilm both taken into account. As a result, a highly reliable,high-performance MOSFET is realized.

[0025] An inventive semiconductor device includes a gate insulating filmthat includes: a first metal oxide film deposited on a semiconductorsubstrate; and a second metal oxide film deposited on the first metaloxide film. In this device, the first and second metal oxide films aremade of the same type of metal oxide and have mutually different oxygenconcentrations.

[0026] In the inventive device, the structure of the gate insulatingfilm has been optimized with the reliability and the dielectric constantof the gate insulating film both taken into account. Thus, the device isimplementable as a highly reliable, high-performance MOSFET.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIGS. 1A through 1D are cross-sectional views showing respectiveprocess steps of a semiconductor device fabricating method according toa first embodiment of the present invention.

[0028]FIG. 2 shows variations in flow rates of Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time in themethod of the first embodiment.

[0029]FIG. 3 is a graph showing the oxygen concentration in thethickness direction of Zr oxide films deposited by the method of thefirst embodiment.

[0030]FIG. 4 is a graph showing the oxygen concentrations in thethickness direction of Zr oxide films deposited by a semiconductordevice fabricating method according to a modified example of the firstembodiment.

[0031]FIG. 5 shows variations in flow rates of Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time in asemiconductor device fabricating method according to a second embodimentof the present invention.

[0032]FIG. 6 shows variations in flow rates of Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time in asemiconductor device fabricating method according to a third embodimentof the present invention.

[0033]FIG. 7 is a cross-sectional view showing a known method forfabricating a semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] Embodiment 1

[0035] Hereinafter, a semiconductor device and a method for fabricatingthe device according to a first embodiment of the present invention willbe described as being applied to an NMOSFET with reference to thedrawings.

[0036]FIGS. 1A through 1D are cross-sectional views showing respectiveprocess steps for fabricating a semiconductor device according to thefirst embodiment. In this method, a sputtering system of a single waferprocessing type is used.

[0037] First, as shown in FIG. 1A, a target 10 of Zr is placed in achamber (not shown) and a dummy silicon substrate 20 is loaded thereto.Then, a reactive sputtering process is provisionally performed using theZr target 10 with a mixed gas ambient containing Ar and O₂ gases createdin the chamber. In this manner, a Zr oxide layer 11 having a thicknessof about 5 nm to about 10 nm is formed in the surface region of the Zrtarget 10. In the meantime, a Zr oxide film 21 is deposited to athickness of about 10 nm to about 100 nm over the dummy siliconsubstrate 20.

[0038] Next, the dummy silicon substrate 20 is unloaded out of thechamber. At this time, the surface of the Zr target 10 is not cleaned bysputtering performed in an Ar gas ambient, for example. Thus, the Zroxide layer 11 remains in the surface of the Zr target 10.

[0039] Then, as shown in FIG. 1B, a p-silicon substrate 30 is loadedinto the chamber. In this case, an isolation region 31 has been definedbeforehand in the surface of the p-silicon substrate 30 by a knownmethod.

[0040] After that, a sputtering process is performed using the Zr target10, including the Zr oxide layer 11 in its surface region, for aboutthree seconds with an Ar gas ambient created in the chamber. In thismanner, a first Zr oxide film 32 is deposited to a thickness of about 1to 2 nm over the p-silicon substrate 30. In this process step, the Zroxide layer 11 in the surface of the Zr target 10 is subjected to thesputtering process, thereby depositing the first Zr oxide film 32 overthe substrate 30. Thus, the first Zr oxide film 32 can be depositedwithout performing a reactive sputtering process in an ambientcontaining an O₂ gas. Accordingly, no silicon dioxide film will beformed between the p-silicon substrate 30 and first Zr oxide film 32. Inaddition, since the first Zr oxide film 32 has a thickness of at mostabout 1 to 2 nm, the Zr oxide layer 11 still remains in the surface ofthe Zr target 10 even after the first Zr oxide film 32 has beendeposited.

[0041] Subsequently, the gas discharge is continued for the sputteringpurpose from the process step shown in FIG. 1B (which will be hereinreferred to as the “first Zr oxide deposition step”), while an O₂ gas isintroduced into the chamber with the Ar gas, which has been used for thefirst Zr oxide deposition step, left in the chamber. Then, as shown inFIG. 1C, a reactive sputtering process can be performed using the Zrtarget 10 with a mixed gas ambient containing Ar and O₂ gases created inthe chamber. As a result, a second Zr oxide film 33 is deposited to athickness of about 3 to 5 nm over the first Zr oxide film 32. That is tosay, the first Zr oxide deposition step and the process step shown inFIG. 1C (which will be herein referred to as the “second Zr oxidedeposition step”) can be performed continuously.

[0042] In the first embodiment, the second Zr oxide film 33 is depositedto have the same oxygen concentration as the first Zr oxide film 32 bycontrolling the flow rate of the O₂ gas during the second Zr oxidedeposition step, for example. Specifically, ZrO₂ films with thestoichimetric composition, for example, may be deposited as the firstand second Zr oxide films 32 and 33 so that the oxygen concentrations ofthe first and second Zr oxide films 32 and 33 are equal to each other,i.e., about 66 at %.

[0043] Then, the p-silicon substrate 30 is unloaded out of the chamber.At this time, the surface of the Zr target 10 is not cleaned by asputtering process performed in an Ar gas ambient, for example. Thus,the Zr oxide layer 11 remains on the surface of the Zr target 10.Accordingly, process steps similar to the first and second Zr oxidedeposition steps can be performed without performing the process stepshown in FIG. 1A (which will be herein referred to as a “targetoxidation step”). That is to say, after at least one dummy silicon waferhas been subjected to the target oxidation step using a sputteringsystem of a single wafer processing type, Zr oxide films to be a gateinsulating film can be deposited over multiple silicon wafers numeroustimes without allowing any silicon dioxide film to exist under the Zroxide films. Accordingly, it is possible to form the gate insulatingfilm consisting essentially of the high-dielectric-constant materialalone while minimizing the time for performing various process stepsother than the steps of forming the gate insulating film and the numberof dummy wafers. That is to say, the gate insulating film consistingessentially of the high-dielectric-constant material alone can be formedat a high throughput and at a low cost.

[0044] After that, as shown in FIG. 1D, a gate electrode 35 is formedover the p-silicon substrate 30 with the gate insulating film 34, madeup of the first and second Zr oxide films 32 and 33, interposedtherebetween. Then, an insulating sidewall 36 is formed on the sidefaces of the gate electrode 35. Subsequently, a doped layer 37 to besource/drain regions is defined in the surface regions of the p-siliconsubstrate 30. Thereafter, an interlevel dielectric film 38 is depositedover the p-silicon substrate 30 as well as over the gate electrode 35.Then, interconnects 39 including plugs, which are formed in theinterlevel dielectric film 38 and connected to the doped layer 37, areformed on the interlevel dielectric film 38, thereby completing anNMOSFET.

[0045]FIG. 2 shows variations in flow rates of Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time for aninterval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the firstembodiment.

[0046] As shown in FIG. 2, the flow rate of the Ar gas is kept constantfrom the start point of the first Zr oxide deposition step through theend point of the second Zr oxide deposition step. On the other hand, theO₂ gas starts to be introduced at the end point of the first Zr oxidedeposition step, i.e., the start point of the second Zr oxide depositionstep. Then, after the flow rate of the O₂ gas has reached apredetermined value (i.e., the mixture ratio of the Ar and O₂ gases isfixed), the flow rate of the O₂ gas will be kept constant until thesecond Zr oxide deposition step is over. In this case, the mixture ratioof the Ar and O₂ gases (in the steady state) is not limited to aparticular value. For example, the ratio of the flow rate of the Ar gasto that of the O₂ gas may range from about 7:3 to about 1:9.

[0047] As also shown in FIG. 2, the gas-discharge power for sputteringis kept constant from the start point of the first Zr oxide depositionstep through the end point of the second Zr oxide deposition step.

[0048] It should be noted that when the second Zr oxide deposition stepis over, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

[0049] In the first embodiment, the flow rate of the O₂ gas isrelatively low at the initial stage of the second Zr oxide depositionstep (see FIG. 2). However, since the Zr oxide layer 11 has been formedin the surface region of the Zr target 10, the lower part of the secondZr oxide film 33 can al-also have a predetermined oxygen concentrationthrough the sputtering of the Zr oxide layer 11. Accordingly, the oxygenconcentration of the second Zr oxide film 33 can be uniformized in thethickness direction.

[0050] Also, in the first embodiment, oxygen defects might exist in thefirst Zr oxide film 32 at the start point of the second Zr oxidedeposition step. However, these oxygen defects disappear when exposed toa plasma created from the O₂ gas during the second Zr oxide depositionstep. Thus, the quality of the first Zr oxide film 32 does not degrade.

[0051]FIG. 3 shows the oxygen concentration in the thickness directionof the first and second Zr oxide films 32 and 33 formed by the method ofthe first embodiment. In FIG. 3, the thickness is measured from thesurface of the p-type silicon substrate 30.

[0052] As shown in FIG. 3, the first and second Zr oxide films 32 and 33have the same oxygen concentration. Specifically, the oxygenconcentration of the films 32 and 33 is set equal to a predeterminedvalue C (which is uniform in the thickness direction).

[0053] As described above, according to the first embodiment, asputtering process is performed using a Zr target 10, including a Zroxide layer 11 in its surface region, with an Ar gas ambient created ina chamber. In this manner, a first Zr oxide film 32 to be the lower partof a gate insulating film 34 is deposited over a p-silicon substrate 30.After that, a reactive sputtering process is performed using the same Zrtarget 10 with a mixed gas ambient containing Ar and O₂ gases created inthe chamber. In this manner, a second Zr oxide film 33 to be the middleor upper part of the gate insulating film 34 is deposited over the firstZr oxide film 32. Accordingly, in the first Zr oxide deposition step,i.e., the initial stage of the process for forming the gate insulatingfilm, no reactive sputtering process is performed but a sputteringprocess is performed using the Zr target 10, including the Zr oxidelayer 11 in its surface region, in an ambient containing no O₂ gas. As aresult, the first Zr oxide film 32 can be deposited over the p-siliconsubstrate 30 without allowing any silicon dioxide film to exist on thesurface of the p-silicon substrate 30. In the second Zr oxide depositionstep, i.e., after the initial stage of the process for forming the gateinsulating film is over, a reactive sputtering process is performed inan ambient containing an O₂ gas with the surface of the p-siliconsubstrate 30 covered with the first Zr oxide film 32. Thus, the secondZr oxide film 33 can be deposited over the first Zr oxide film 32without allowing any silicon dioxide film to exist on the surface of thep-silicon substrate 30. Accordingly, the gate insulating film can beessentially made up of the first and second Zr oxide films 32 and 33alone. In other words, a gate insulating film consisting essentially ofa high-dielectric-constant material alone can be formed. As a result,the resultant MOSFET can have its gate capacitance increased and itsdrivability enhanced. Furthermore, a gate leakage current can beminimized because the gate insulating film can be thick enough with adesired gate capacitance maintained.

[0054] Also, according to the first embodiment, before the p-siliconsubstrate 30 is loaded into the chamber, a reactive sputtering processis performed provisionally using an unoxidized Zr target 10 with a mixedgas ambient containing Ar and O₂ gases created in the chamber. In thismanner, the surface region of the Zr target 10 can be oxidized easily.In this process step, the provisional reactive sputtering is performedon a dummy silicon substrate 20 that has been loaded into the chamberbeforehand. Thus, no insulating Zr oxide is deposited on a wafer stage(which will be used as a gas-discharge electrode during the sputteringprocess) in the chamber when the Zr target 10 is oxidized. As a result,it is possible to avoid the inability to apply a voltage to thep-silicon substrate 30 in subsequent process steps.

[0055] In addition, according to the first embodiment, the second Zroxide deposition step includes the step of continuing the gas dischargefor the sputtering purpose from the first Zr oxide deposition step,while introducing an O₂ gas into the chamber with the Ar gas, used forthe first Zr oxide de-deposition step, left in the chamber. Accordingly,the first and second Zr oxide deposition steps can be performedcontinuously. As a result, the throughput of the process improves.

[0056] In the first embodiment, the Ar gas is used as an inert gas.However, the same effects are attainable even if Xe (xenon) or any otherinert gas is used instead of Ar gas.

[0057] Also, in the first embodiment, the Zr target 10 is used.Alternatively, any other metal such as Hf, La, Ta or Al, which canproduce an oxide with a high dielectric constant through a reactivesputtering process, may be used for the target because similar effectsare achieved with any of these metals.

[0058] Further, in the first embodiment, the Zr oxide layer 11 is formedin the surface region of the Zr target 10 by performing the reactivesputtering process provisionally using the unoxidized Zr target 10 withthe mixed gas ambient containing the Ar and O₂ gases created in thechamber. Alternatively, a target of a metal such as Zr, at least thesurface region of which has been oxidized, may be prepared in thechamber.

[0059] In addition, in the first embodiment, Zr oxide films are supposedto make up the gate insulating film 34. Alternatively, a Zr oxide filmcontaining Si or any other metal oxide film containing Si may be used asthe gate insulating film 34. Then, the lattice strain created in part ofthe p-silicon substrate 30 in contact with the gate insulating film 34can be relaxed. As a result, decrease in carrier mobility can besuppressed. In this case, Si may be added to a metal oxide film such asZr oxide film to be the gate insulating film by performing a reactivesputtering process using a metal target of, e.g., Zr containing Si.Alternatively, a reactive sputtering process may be performed using atarget of Si as well as a target of a metal such as Zr.

[0060] Furthermore, in the first embodiment, the oxygen concentration ofthe second Zr oxide film 33 is controlled by adjusting the flow rate ofthe O₂ gas in the second Zr oxide deposition step. But the oxygenconcentration of the first Zr oxide film 32 may be controlled byadjusting the flow rate of the O₂ gas during the target oxidation step(the process step shown in FIG. 1A), i.e., by adjusting the oxygenconcentration of the Zr oxide layer 11 formed in the surface region ofthe Zr target 10.

[0061] Modified Example of Embodiment 1

[0062] Hereinafter, a semiconductor device and a method for fabricatingthe device according to a modified example of the first embodiment ofthe present invention will be described with reference to the drawings.

[0063] The method of this modified example is different from the methodof the first embodiment in the oxygen concentrations of the first andsecond Zr oxide films 32 and 33.

[0064] Specifically, in the first embodiment, the second Zr oxide film33 is deposited to have the same oxygen concentration as that of thefirst Zr oxide film 32 as shown in FIG. 3, for example.

[0065] On the other hand, in the modified example of the firstembodiment, the second Zr oxide film 33 is deposited to have a differentoxygen concentration from that of the first Zr oxide film 32 bycontrolling the flow rate of the O₂ gas during the second Zr oxidedeposition step, for example.

[0066]FIG. 4 shows the oxygen concentrations in the thickness directionof the first and second Zr oxide films 32 and 33 deposited by the methodof the modified example of the first embodiment. In FIG. 4, thethickness is measured from the surface of the p-type silicon substrate30.

[0067] As shown in FIG. 4, the first Zr oxide film 32 has an oxygenconcentration higher than that of the second Zr oxide film 33.Specifically, the oxygen concentration of the first Zr oxide film 32 isset to a first predetermined value C1 (which is constant in thethickness direction). On the other hand, the oxygen concentration of thesecond Zr oxide film 33 is set to a second predetermined value C2, whichis also constant in the thickness direction. In this case, C1>C2.

[0068] According to the modified example of the first embodiment, thefollowing effects are attained as well as those obtained by the methodof the first embodiment.

[0069] Specifically, the second Zr oxide film 33 is deposited to have adifferent oxygen concentration from that of the first Zr oxide film 32by controlling the flow rate of the O₂ gas during the second Zr oxidedeposition step. Thus, the structure of the resultant gate insulatingfilm 34, made up of the first and second Zr oxide films 32 and 33, canbe optimized with the reliability and the dielectric constant of thegate insulating film 34 both taken into account. As a result, a highlyreliable, high-performance MOSFET is realized.

[0070] In this modified example, the first Zr oxide film 32 has anoxygen concentration higher than that of the second Zr oxide film 33 asshown in FIG. 4. Alternatively, the oxygen concentration of the first Zroxide film 32 may be lower than that of the second Zr oxide film 33.

[0071] Further, in this example, the oxygen concentration of the secondZr oxide film 33 is controlled by adjusting the flow rate of the O₂ gasduring the second Zr oxide deposition step. But the oxygen concentrationof the first Zr oxide film 32 may be controlled by adjusting the flowrate of the O₂ gas during the target oxidation step, i.e., by adjustingthe oxygen concentration of the Zr oxide layer 11 formed in the surfaceregion of the Zr target 10.

[0072] Embodiment 2

[0073] Hereinafter, a semiconductor device and a method for fabricatingthe device according to a second embodiment of the present inventionwill be described with reference to the drawings.

[0074] The method of the second embodiment is different from the methodof the first embodiment in when the second Zr oxide deposition step (seeFIG. 1C) is started after the first Zr oxide deposition step (see FIG.1B) is over.

[0075] Specifically, in the first embodiment shown in FIG. 2, even afterthe first Zr oxide deposition step is over, the gas-discharge iscontinued for the sputtering purpose, while the O₂ gas is introducedinto the chamber with the Ar gas, which has been used for the first Zroxide deposition step, left in the chamber. In this manner, the secondZr oxide deposition step, or the process step of depositing the secondZr oxide film 33 over the first Zr oxide film 32, is performed. That isto say, in the first embodiment, the first and second Zr oxidedeposition steps are performed continuously.

[0076] On the other hand, in the second embodiment, the gas-dischargefor sputtering is suspended during the interval between the first andsecond Zr oxide deposition steps. In the meantime, the O₂ gas isintroduced into the chamber with the Ar gas, which has been used for thefirst Zr oxide deposition step, left in the chamber. In this case, thegas-discharge is suspended until the flow rate of the O₂ gas introducedinto the chamber reaches, and settles at a predetermined value, e.g.,for about 10 to 15 seconds. When the flow rate of the O₂ gas reaches thepredetermined value, i.e., when the mixture ratio of the Ar and O₂ gasesis fixed, the gas-discharge is resumed, thus starting the second Zroxide deposition step. That is to say, in the second embodiment, a nodischarge interval for changing the ambient in the chamber is placedbetween the first and second Zr oxide deposition steps.

[0077]FIG. 5 shows variations in flow rates of the Ar and O₂ gases withtime and a variation in gas-discharge power for sputtering with time foran interval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the secondembodiment.

[0078] As shown in FIG. 5, the flow rate of the Ar gas is kept constantfrom the beginning of the first Zr oxide deposition step through the endof the second Zr oxide deposition step. On the other hand, the O₂ gasstarts to be introduced when the first Zr oxide deposition step is over.Then, on and after the mixture ratio of the Ar and O₂ gases is fixed atthe start point of the second Zr oxide deposition step, the flow rate ofthe O₂ gas will be kept constant until the second Zr oxide depositionstep is over. In this case, the mixture ratio of the Ar and O₂ gases (inthe steady state) is not limited to any particular value. For example,the ratio of the flow rate of the Ar gas to that of the O₂ gas may rangefrom about 7:3 to about 1:9.

[0079] As also shown in FIG. 5, the gas-discharge power for sputteringis kept constant while each of the first and second Zr oxide depositionsteps is being performed. But the gas-discharge power is kept off (i.e.,reduced to zero) for the interval between the first and second Zr oxidedeposition steps.

[0080] It should be noted that when the second Zr oxide deposition stepis over, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

[0081] According to the second embodiment, the following effects areattained as well as those obtained by the first embodiment.

[0082] Specifically, in the interval between the first and second Zroxide deposition steps, the gas-discharge for sputtering is suspended,while the O₂ gas is introduced into the chamber with the Ar gas, whichhas been used for the first Zr oxide deposition step, left in thechamber. Thus, the mixture ratio of the Ar and O₂ gases can be fixedbefore the second Zr oxide deposition step is started. As a result, theoxygen concentration of the second Zr oxide film 33 is controllable moreeasily.

[0083] For the second embodiment, the first and second Zr oxide films 32and 33 may have the same oxygen concentration as in the first embodiment(see FIG. 3). Alternatively, as in the modified example of the firstembodiment, the first and second Zr oxide films 32 and 33 may havemutually different oxygen concentrations (see FIG. 4).

[0084] Embodiment 3

[0085] Hereinafter, a semiconductor device and a method for fabricatingthe device according to a third embodiment of the present invention willbe described with reference to the drawings.

[0086] The method of the third embodiment is different from that of thefirst embodiment in the following two respects. Firstly, the second Zroxide deposition step (see FIG. 1C) is started at a different time afterthe first Zr oxide deposition step (see FIG. 1B) is over. Secondly, themixture of the Ar and O₂ gases is produced in a different manner for thesecond Zr oxide deposition step.

[0087] Specifically, in the first embodiment shown in FIG. 2, even afterthe first Zr oxide deposition step is over, the gas-discharge iscontinued for the sputtering purpose, while the O₂ gas is introducedinto the chamber with the Ar gas, which has been used for the first Zroxide deposition step, left in the chamber. In this manner, the secondZr oxide deposition step, or the process step of depositing the secondZr oxide film 33 over the first Zr oxide film 32, is performed. That isto say, in the first embodiment, the first and second Zr oxidedeposition steps are performed continuously. In addition, the Ar gasthat has been used for the first Zr oxide deposition step is reused aspart of the mixture for use in the second Zr oxide deposition step.

[0088] On the other hand, in the third embodiment, the gas-discharge forsputtering is suspended during the interval between the first and secondZr oxide deposition steps. In the meantime, the Ar gas that has beenused for the first Zr oxide deposition step is exhausted from thechamber, and then an Ar gas is newly introduced along with the O₂ gasinto the chamber. In this case, the gas-discharge is suspended until therespective flow rates of the Ar and O₂ gases introduced into the chamberreach, and are settled at, predetermined values (which may be mutuallydifferent), e.g., for about 10 to 15 seconds. When the flow rates of theAr and O₂ gases reach the predetermined values, i.e., when the mixtureratio of the Ar and O₂ gases is fixed, the gas-discharge is resumed,thus starting the second Zr oxide deposition step. That is to say, inthe third embodiment, a no discharge interval for changing the ambientin the chamber is placed between the first and second Zr oxidedeposition steps. In addition, the Ar gas is newly introduced as part ofthe mixture for use in the second Zr oxide deposition step withoutreusing the Ar gas that has been used for the first Zr oxide depositionstep.

[0089]FIG. 6 shows variations in flow rates of the Ar and O₂ gases withtime and a variation in gas-discharge power for sputtering with time foran interval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the thirdembodiment.

[0090] As shown in FIG. 6, the flow rate of the Ar gas is kept constantwhile the first Zr oxide deposition step is being performed. However,once the first Zr oxide deposition step is over, the Ar gas is exhaustedfrom the chamber so as to have its flow rate reduced to zero before thesecond Zr oxide deposition step is started. On the other hand, the O₂and new Ar gases start to be introduced into the chamber after the firstZr oxide deposition step is over. Then, on and after the mixture ratioof the Ar and O₂ gases has been fixed at the start point of the secondZr oxide deposition step, the flow rates of the Ar and O₂ gases are keptconstant until the second Zr oxide deposition step is over. In thiscase, the mixture ratio of the Ar and O₂ gases (in the steady state) isnot limited to any particular value. For example, the ratio of the flowrate of the Ar gas to that of the O₂ gas may range from about 7:3 toabout 1:9.

[0091] As also shown in FIG. 6, the gas-discharge power for sputteringis kept constant while each of the first or second Zr oxide depositionsteps is being performed. But the gas-discharge power is kept off (i.e.,reduced to zero) for the interval between the first and second Zr oxidedeposition steps.

[0092] It should be noted that when the second Zr oxide deposition stepis over, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

[0093] According to the third embodiment, the following effects areattained as well as those obtained by the first embodiment.

[0094] Specifically, during the interval between the first and second Zroxide deposition steps, the gas-discharge for sputtering is suspended.In the meantime, the Ar gas used for the first Zr oxide deposition stepis exhausted from the chamber, and then an Ar gas is newly introducedalong with the O₂ gas into the chamber. Thus, the mixture ratio of theAr and O₂ gases should be fixed before the second Zr oxide depositionstep is started. As a result, the oxygen concentration of the second Zroxide film 33 is controllable much more easily.

[0095] For the third embodiment, the first and second Zr oxide films 32and 33 may have the same oxygen concentration as in the first embodiment(see FIG. 3). Alternatively, as in the modified example of the firstembodiment, the first and second Zr oxide films 32 and 33 may havemutually different oxygen concentrations (see FIG. 4).

[0096] Also, in the third embodiment, after the first Zr oxidedeposition step is over, the new Ar gas is introduced along with the O₂gas into the chamber. In this case, the Ar and O₂ gases may beseparately introduced into the chamber and then mixed together therein.Alternatively, the Ar and O₂ gases may be pre-mixed together outside ofthe chamber and then introduced into the chamber. In the latter case,the mixture ratio of the Ar and O₂ gases can be fixed in a shorter time.Thus, the no discharge interval between the first and second Zr oxidedeposition steps can be shortened, thereby improving the throughput ofthe process.

What is claimed is:
 1. A method for fabricating a semiconductor device,the method comprising the steps of: a) preparing a metal target in achamber, at least a surface region of the target having been oxidized;b) performing a sputtering process using the metal target with an inertgas ambient created in the chamber, thereby depositing a first metaloxide film as a lower part of a gate insulating film over asemiconductor substrate; and c) performing a reactive sputtering processon the metal target with a mixed gas ambient, containing the inert gasand an oxygen gas, created in the chamber, thereby depositing a secondmetal oxide film as a middle or upper part of the gate insulating filmover the first metal oxide film.
 2. The method of claim 1, wherein thestep a) comprises the step of performing a provisional reactivesputtering process on the metal target to be oxidized with a mixed gasambient, containing the inert and oxygen gases, created in the chamber,thereby oxidizing the surface region of the metal target before thesemiconductor substrate is loaded into the chamber.
 3. The method ofclaim 2, wherein the provisional reactive sputtering process isperformed on another semiconductor substrate that has been loaded intothe chamber before the step a) is started.
 4. The method of claim 1,wherein the step c) comprises the step of introducing the oxygen gasinto the chamber with the inert gas, used in the step b), left in thechamber, and with a gas-discharge continued from the step b) to carryout the reactive sputtering process.
 5. The method of claim 1, furthercomprising, between the steps b) and c), the step of introducing theoxygen gas into the chamber with the inert gas, used in the step b),left in the chamber, and with a gas-discharge for the sputtering processsuspended.
 6. The method of claim 1, further comprising, between thesteps b) and c), the step of exhausting the inert gas, used in step b),from the chamber and then newly introducing the inert gas along with theoxygen gas into the chamber with a gas-discharge for the sputteringprocess suspended.
 7. The method of claim 1, wherein the step c)comprises the step of supplying the oxygen gas at a controlled flow rateinto the chamber to deposit the second metal oxide film with a differentoxygen concentration from that of the first metal oxide film.
 8. Asemiconductor device comprising a gate insulating film that includes: afirst metal oxide film deposited on a semiconductor substrate; and asecond metal oxide film deposited on the first metal oxide film, whereinthe first and second metal oxide films are made of the same type ofmetal oxide, and wherein the first and second metal oxide films havemutually different oxygen concentrations.